Studies are being conducted to investigate the immunobiology of human immunodeficiency virus (HIV-1) infection. Cells (monocytes and T lymphocytes) that bear the CD4+ molecules are the primary targets for infection of HIV-1. In vitro, certain isolates preferentially infect one cell type or the other. Cell-free vs. cell to cell infection was compared using monocytotropic (BAL) and T cell tropic (HTLVIIIB) strains. T cell infection was 10 to 15 times more efficient with HTLV-IIIB; however, virus production by the BAL-infected monocytes was increased by this order of magnitude resulting in equivalent levels of infection in T cells by cell to cell infection. Studies are underway to characterize the virus-like particle(s) seen in Kaposi's sarcoma tissue biopsies from individuals in a cohort of HIV-1, HIV-2, HTLV-I, and HTLV-II seronegative Greek individuals. Sera from these individuals, as well as from normal individuals, were tested by Western blot techniques for reactivity to nonhuman primate retroviruses. Reactions were observed in some instances to gag and transmembrane proteins of the type D viruses with sera from Kaposi's patients. Studies are underway to investigate the nature of the antibody response to HIV-1 proteins in infected mothers and their infants. The antibody response in these individuals was varied with most frequent reactions of the IgGl subclass with p24, gpl2O, and gpl6O. Other findings included the detection of IgA, IgD, and IgE antibodies to various viral proteins.